The present invention is directed to electrostatic coating, and more particularly to improved systems for the application of conductive coating materials by electrostatic techniques.
Electrostatic paint spraying techniques are, in general, well known and involve the discharge of coating material in atomized form, with the atomized particles carrying an electrostatic charge and with the workpiece being electrically charged to an opposite polarity in order to establish an electrostatic attraction for the charged atomized particles. Typically, the paint atomizing equipment is charged to a high voltage relative to the workpiece, with voltages in excess of 100,000 volts being common. Accordingly, it is necessary to carefully isolate the electrically charged components, not only for safety purposes, but also to prevent the charge from being drained off to ground. This can present a considerable problem when the coating material is of a conductive nature, particularly where the coating material uses water as a solvent, for example, or where the coating material, even though utilizing a nonconductive solvent, incorporates a conductive pigment material, such as with metallic coatings.
One of the known procedures for electrically isolating conductive coating materials is to contain the coating material within a pressurized, electrically isolated container, which is connected to the atomizing device. This arrangement, while satisfactory for batch operations, does not lend itself to the operation of continuous painting lines, nor does it accommodate expeditious color change where items being conveyed along a paint line conveyor, for example, are selectively painted with different colors according to production requirements.
In accordance with the present invention, a novel and highly simplified system is provided which permits the utilization of condutive coating materials and which accommodates the continuous operation of the paint line. The system includes a voltage block arrangement which, in effect, temporarily isolates from the system a container of electrically charged, conductive coating material, yet which permits the container to be periodically replenished, as necessary, in order to maintain continuous operation of the paint line but at the same time maintaining electrical isolation of the highly charged paint vessel from the primary paint supply. The system of the invention enables the primary supply to be in the form of a recirculating system, such that the primary paint materials are continuously being circulated back to a paint room and mixing area, to avoid stagnation and sedimentation of the paint. Moreover, the system of the invention easily accommodates rapid color changes.
In one advantageous form of the invention, the voltage block system includes a pair of associated tanks or vessels for the coating material. One of the vessels, referred to as an inventory tank, maintains a supply of coating material constantly under pressure and continuously being supplied to the atomizing devices. The second tank, referred to herein as a transfer tank, is alternatively connectable to the inventory tank or to the primary paint supply. During a painting operation, the transfer tank is temporarily connected to the main paint supply and filled with a predetermined batch of coating material. When the inventory tank becomes depleted, the transfer tank is connected to the inventory tank, while being isolated from the main supply, and coating material is caused to be delivered from the transfer tank into the inventory tank in order to maintain continuity of supply to the atomizing devices.
In another and highly simplified form of the invention, a single inventory tank or vessel is provided, which is calculated to retain an appropriate volume of coating material to complete a single unit, for example (or a predetermined number of units, if desired). During the inverval between units, as they are carried along by means of a continuous conveyor system, the high voltage electrostatic charging system is momentarily disabled, while the inventory tank is connected to the main supply system and replenished with a predetermined volume of material. As soon as the replenishment operation has been completed, the inventory tank is again isolated from the main supply system and the high voltage charging system is reenergized.
In any of its basic forms, the system of the invention advantageously utilizes air or fluid pressure to effect the transfer of the coating material from the transfer vessel to the inventory vessel and for the control of the levels of coating material within the vessel. Control systems provided for this purpose are simple and highly reliable for the service conditions under which the system is typically operated.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and to the accompanying drawings.